202 DETERMINATION OF ALKALIES IN MINERALS. 



heat of an argand lamp, and when the mass begins to agglutinate the crucible 

 should be closed and its cover tied down with platinum wire, then placed in 

 a Hessian crucible closed up also; the whole is placed upon an inverted 

 crucible and submitted to the action of the blast of a wind-furnace, begin- 

 ning first gradually with a red heat, piling on more coke, so as to fill up the 

 furnace, and increasing the heat to the highest possible pitch, until the Hes- 

 sian crucible begins to soften. It is absolutely necessary to the success of the 

 operation that the Hessian crucible should be closed as well as possible, which 

 is best done by luting the cover with fire-clay; the Hessian crucible and its 

 cover, having fused together, can not be separated except by breaking, etc." 



It will be seen in reading this extract that the heat required 

 is not ordinarily at the command of most chemists; in fact, no 

 other variety of furnace than a Sefstroem can be depended on 

 for a complete decomposition. 



10. Caustic lime and its salts have also been recommended 

 and long used for the more imperfect decomposition of silicates, 

 as for obtaining lithia from spodumene and lepidolite. Lime 

 or its carbonate, well mixed with many silicates finely pulver- 

 ized, will decompose them completely at a white heat, but no 

 one salt of lime is capable of meeting the demand of the entire 

 range of alkaline silicates. 



11. In consideration of these difficulties Berzelius proposed 

 the use of hydrofluoric acid, and this method, when applied 

 with the numerous precautions required, will serve to decom- 

 pose all silicates; still, according to Eose, there are siliceous 

 compounds that can. not be completely decomposed by hydro- 

 fluoric acid. Besides, this acid is a most disagreeable one to 

 manipulate with, whether we employ Brunner's apparatus or 

 Laurent's method, or, what is always the best, the concentrated 

 acid previously prepared. I may also add that the necessity 

 of using sulphuric acid after the decomposition is made is 

 another objectionable feature in this process. 



12. The above furnishes a hasty review of the methods we 

 are now possessed of for decomposing the silicates in order to 

 determine their alkalies ; their merits can be contrasted with 

 those of the method about to be described. 



13. The decomposing agent which I present as a substitute 

 for all others, and as capable of meeting the demands proposed 

 in the commencement of this article, is a mixture of carbonate 

 of lime and fluor-spar. 



14. Carbonate of lime I have used for more than six years 



